Identify the correct molecular picture showing what happens at the critical micellar concentration (CMC) of an aqueous solution of a surfactant.

To understand what happens at the Critical Micellar Concentration (CMC), we examine the behavior of surfactant molecules in an aqueous solution.

Surfactant molecules are amphiphilic, consisting of a polar hydrophilic head and a non-polar hydrophobic tail. At concentrations below the CMC, the surfactant molecules preferentially adsorb at the air-water interface, with their hydrophilic heads immersed in water and their hydrophobic tails directed away from it. This arrangement lowers the surface tension of the solution.

As the concentration of surfactant increases, the surface eventually becomes saturated with surfactant molecules. This concentration is known as the Critical Micellar Concentration (CMC). Beyond this point, additional surfactant molecules can no longer be accommodated at the surface.

To minimize the contact between their hydrophobic tails and water, the excess surfactant molecules aggregate in the bulk solution to form micelles. In a micelle, the hydrophobic tails are oriented inward, away from water, while the hydrophilic heads remain on the outside in contact with the surrounding water.

Among the given options:

• Option (A) represents surfactant molecules adsorbed only at the surface and corresponds to concentrations below the CMC.
• Options (B) and (C) do not depict the correct arrangement of molecules in a stable micelle.
• Option (D) correctly shows a saturated surface along with the formation of a micelle in the bulk solution, where the hydrophobic tails point inward and the hydrophilic heads point outward.

Hence, the correct representation of the system at the Critical Micellar Concentration is Option D.